The embodiments described herein relate generally to an electric machine drive system, and more particularly, to reducing a level of common mode current applied to ground by an electric machine drive system.
A three phase inverter may be used to drive an electric machine, for example, an induction motor or a permanent magnet motor. Such an inverter typically provides favorable control and efficiency. However, the switching pattern of power semiconductor devices typically included in the inverter may generate an electromagnetic interference (EMI) current. Using shielded cables to couple the inverter and the electric motor reduces the EMI current, including a common mode current component of the EMI current. Unfortunately, shielded cables are not suitable for use in all applications. Common mode inductors may also be used to reduce common mode current, except that size constraints may prevent use of such inductors. An active circuit that injects a common mode current in a DC input side of the inverter may be used to condition the DC power applied to the inverter. Such a circuit, though, does not reduce the common mode current present at the electric motor, especially when the inverter and electric motor are coupled by a long unshielded cable. Furthermore, a controlled voltage source may be included at the AC output of the inverter to cancel the common mode voltage generated by the inverter, and therefore reduce the common mode current. Reducing the common mode current in this manner, however, requires a transformer in series with the cable that couples the inverter to the electric motor.